In general, when advertising films and structural exterior materials are exposed to outdoor environments for a long time, there are serious problems in that the films and materials are contaminated by dust, smoke and exhaust gas and discolored by ultraviolet rays. Therefore, it is particularly required for such outdoor films and exterior materials to show physical properties such as self-cleaning property and weather resistance with long-term reliability.
Contamination on surfaces of outdoor films and structural exterior materials mainly results from dust in the air, combustion products such as carbon black and inorganic materials such as clay particles. Generally, contaminants deposited on surfaces of outdoor films and structural exterior materials may be cleaned by rain intermittently, so that the films and materials recover their original conditions. However, in the case of a fine rain, such contaminants are captured by the rain and adhered to the surfaces. Then, such adhered contaminants are present on the surfaces in the form of stripes of dust, contaminants and stains while they dry. Additionally, during the dry season, generation of pollutants in the air increases due to the lack of rain. Any cleaning effect from rain cannot be obtained, resulting in continuous deposition of contaminants. Therefore, coating materials having staining resistance have been conventionally used in order to solve the above problems.
The most widely used method comprises using photocatalyst particles. When the surface of photocatalyst particles is irradiated with ultraviolet rays, the surface shows strong hydrophilicity. Although such a strongly hydrophilic state is maintained for several hours to one-week even if the irradiation is stopped, the surface gradually recovers its original hydrophobic state. Additionally, after the surface recovers the original hydrophobic state, it may regain ultra-hydrophilicity rapidly by irradiating ultraviolet rays again. In other words, intermittent irradiation rather than continuous irradiation is sufficient to maintain surface hydrophilicity and staining resistance. However, such methods using a photocatalyst coating layer are disadvantageous in that they need an additional step of coating a primer on a plastic substrate formed of thermoplastic resin, etc., before coating a photocatalyst, because photocatalysts generally have a possibility for decomposing organic substances.
Additionally, fluoro group-containing water repellent coating materials such as polytetrafluoro ethylene (PTFE) have been considered as preferred staining resistant material. Water repellent coating materials serve to remove dust from a surface layer while water drops on a coating film move downwardly by low surface tension. However, when water drops are too small to be moved by gravity, said water drops are dried along with pollutants, thereby forming local stains.
Further, a method for coating a hydrophilic graft heteropolymer has been suggested. However, the coating film formed by the hydrophilic graft heteropolymer shows a contact angle with water of between 30 and 40 degrees, while conventional mineral dust shows a contact angle with water of between 20 and 50 degrees (Newspaper Daily Chemical Industry, 30, 1995). Therefore, such heteropolymers cannot efficiently clean stains and contaminants formed by mineral dust deposited on surfaces. Additionally, because other easily available hydrophilic coatings have a contact angle with water of between 50 and 70 degrees, they are problematic in that they cannot prevent pollution by dust in the city efficiently.
Meanwhile, it is not known that conventional structural exterior materials (for example, vinyl siding) are available as products having self-cleaning property and staining resistance. A staining resistant coating is required in order to provide such properties. However, because vinyl siding materials are generally firm and rigid, it is difficult to produce them in a large scale as general roll-shaped films
Additionally, structural exterior materials are produced by a production line comprising an extruder, calibration table (for molding and cooling) and a press (for cutting). The production line optionally further comprises additional supplementary units so that such materials are produced by a production line including an extruder, embossing unit (Embo unit), calibration table (for molding and cooling), punching unit, haul-off (for tension control) and a press (for cutting).
In order to impart staining resistance to a structural exterior material, it is necessary to carry out an additional coating step after the above-mentioned production process. Therefore, an additional coating line is also required, thereby reducing production yield and increasing production cost. In other words, the overall process is inadvisable in both terms of cost and working space.